1. Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, is concerned with an improved hydraulic pressure-responsive balance mechanism for maintaining parallelism between relatively rotatable fluid film-riding surfaces of an axial thrust force-compensating apparatus.
2. Description of the Prior Art
Gas turbine engines generally include a gas generator which comprises a compressor for compressing air flowing aft through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a turbine driven by the gas stream and connected for driving a rotor which, in turn, drives the compressor. Many engines further include a second turbine, known as a power turbine, located aft of the gas generator and which extracts energy from the gas flow to drive a rotating load with variable pitch blades such as found in the propulsor of helicopters, ducted turbofan engines, and turboprop engines.
A recent improvement over the turbofan and turboprop engines is an unducted fan engine such as disclosed in the first U.S. patent application cross-referenced above. In the unducted fan engine, the power turbine includes counterrotating rotors with turbine blades defining counterrotating airfoil stages which drive in corresponding fashion unducted fan blades radially located with respect to the power turbine. The fan blades of the unducted fan engine are variable pitched blades to achieve optimum performance. During operation, fuel efficiency of the engine can be increased by varying the pitch of the blade to correspond to specific operating conditions.
In the unducted fan engine, as in many other types of engines, large axial forces are generated on the rotors by reaction of their turbine blades to pressure drops across them. Ball thrust bearings between a stationary engine structure and the outer rotor and between the outer and inner rotors are used to prevent axial displacement of the rotors. However, the maximum load limits of the ball thrust bearings are typically less than the amount of axial thrust force generated by the blades.
A labyrinth gas seal which is used in the unducted fan engine to minimize leakage of pressurized gas flow to outside the flowpath between the rotors and resulting reduction of engine performance, is also employed to take up the additional axial force over the maximum allowable for the ball thrust bearings to protect the bearings. However, the labyrinth seal is a large diameter seal which must have a relatively large radial clearance to accommodate differential thermal growth between opposing components of the seal. The large clearance imposes a high performance penalty on the engine, for instance approaching one percent performance loss, due to relatively large leakage and accompanying loss of energy through the seal.
One way to reduce the performance penalty of this large diameter seal is to reduce its diameter. For example, if the diameter of the seal could be reduced by one-half, gas leakage could be reduced by three-fourths. However, reduction of the labyrinth seal diameter to reduce the performance penalty also reduces the utility of the seal for balancing off the axial thrust forces to protect the ball thrust bearings of the engine.
Consequently, in order to attain improvement in engine performance by reduction of labyrinth seal diameter size, an alternative approach is needed to balance off the axial thrust forces to protect the ball thrust bearings of the engine.